It is common practice to provide in an automobile distributor, automatic means for advancing the firing point of each cylinder ahead of "top dead center", that is, the point at which a maximum amount of burning of fuel in the cylinder occurs. This automatic means causes the amount of advance, which is commonly measured in degrees, to vary in accordance with the engine speed. The amount of advance increases as the engine speed increases, for reasons which are well understood to those skilled in the art.
A device which measures the amount of advance at a given engine speed has a strobe lamp that is triggered by pulses corresponding to the sparks for a selected cylinder. The strobe lamp is aimed at the engine block and the adjacent rotating flywheel thereon. A timing mark on the flywheel appears stationary because the strobe cycling matches the engine speed. One such advance measuring device is disclosed in U.S. Pat. No. 2,715,711, in which operation of the strobe lamp is delayed with respect to the spark event. A knob is rotatable to adjust such delay so that the flash from the strobe lamp occurs at top dead center. The operator can then read the meter to determine the amount of advance, which is precisely equal to the delay.
It has been proposed to reduce the cost of an advance measuring device by eliminating the meter, and calibrating the knob itself so that it displays information on the amount of advance.
The knob in the advance measuring device disclosed in U.S. Pat. No. 2,715,711 cannot be so calibrated. In the circuit disclosed in this patent, rotating the knob causes the amount of resistance in a timing circuit to change. For example, if the spark event leads top dead center by 20 microseconds, then the knob would be rotated to delay the flash 20 microseconds, resulting in a given knob setting. If the speed of the engine were doubled, for example, the time between the spark event and top dead center would be halved to 10 microseconds. To achieve half the delay, the knob would be rotated to a different position, although the amount of advance in engine degrees might have been exactly the same. Thus, gradations cannot simply be added to a knob that adjusts timing alone. The circuitry must take into consideration the speed of the engine.
There have been several prior art devices which do not require a meter, but instead include a knob that is calibrated in degrees of advance. In each of these prior art devices, however, the manner in which engine speed is taken into consideration causes the device to respond slowly to rapid changes in engine speed such as occurs when an engine is running roughly. Another shortcoming of these prior devices is that several seconds are required for the measuring device to "lock in".
One such prior art device is disclosed in U.S. Pat. No. 2,785,215 to Yetter, dated Mar. 12, 1957. Taking FIG. 1 of Yetter as exemplary, a sawtooth wave is generated, applied to one terminal of a diode 24, and compared to the average amplitude of the sawtooth wave as measured by the elements 16 and 18. After a certain number of cycles have been generated, a steady state of DC voltage will be provided for the cathode of the diode 24. It takes a number of cycles to develop this average voltage, and thus several seconds are required before the advance measuring device is in condition to provide useful information. Furthermore, if the engine speed fluctuates, such as when the engine is running roughly, this average voltage does not change quickly because it is an average of a number of cycles. The elements 16 and 18 tend to smooth out these variations. The sawtooth wave on the other hand responds substantially instantly and its characteristics will take into consideration these short-term changes in engine speed. Thus, the timing mark on the flywheel of an engine which is running roughly will not appear stationary when illuminated by the Yetter device.
A second prior art device which is meterless is disclosed in U.S. Pat. No. 3,597,677 to MacCrea et al., dated Aug. 3, 1971. The advance measuring device disclosed in this patent produces a sawtooth wave which is applied to a threshold detector comprising the transistors 107 and 108, the threshold voltage level of which is determined by the setting of a potentiometer 78. This particular circuit takes into consideration variations in engine speed by examining the sawtooth present on the conductor 94. A slower engine speed will result in a tendency of the voltage at that point to increase, thereby increasing the conduction of the transistors 83 and 84, supplying a decreased bias to the transistor 66, thereby raising the effective charging impedance supplied by the transistors 66 and 67. This biasing voltage is smoothed out by virtue of the capacitors 81 and 91. It takes some time for these capacitors to charge up when the advance measuring device is turned on and it is only after such steady state is reached that accurate readings may be taken. Furthermore, if the speed tends to fluctuate such as when the engine is running roughly, these capacitors will not respond quickly but instead will smooth over such changes. So, just as with the device disclosed in Yetter, the lock-in time is relatively slow, and the measuring device does not respond to rapid fluctuations in engine speed.
The MacCrae, et al. patent requires the sawtooth amplitude to be maintained constant, which is disadvantageous since it requires close tolerances to be maintained in the parts and in the B+ supply voltage.
A third prior art device, made by Fox Valley Instrument Co., appears to be unpatented at the present but has been in the marketplace. Enclosed is a print prepared by the assignee of the present application, Snap-On Tools Corporation, based on a model of such device. In the Fox Valley unit, the threshold voltage must follow a very carefully controlled hyperbolic relationship with speed, which relationship is difficult to achieve accurately and is rather expensive. Also, the threshold voltage tends to vary with the supply voltage, thereby requiring that the supply voltage output be closely monitored. A knob, which operates a 0-5 K potentiometer, controls the slope of the rising portion of the sawtooth waveform, the greater the slope, the less the delay.